Hydrogeology and water chemistry of the Great Plains (Dakota, Kiowa,
and Cheyenne) and Cedar Hills Aquifers in Central Kansas

Abstract

Shallow underground disposal of oil-field brines in the Cedar Hills has
prompted concern related to induced upward migration of the brines into
freshwater zones of the Great Plains aquifer in central Kansas. Within
this area, 460 wells are actively being used to dispose of oil-field
brines into the Cedar Hills aquifer at an average rate of 550 bbls. per
day per well.

In order to evaluate the potential for upward migration of these brines,
the Kansas Geological Survey conducted a subregional hydrogeologic
investigation of the Great Plains and Cedar Hills aquifers to assess the
geologic and hydrologic factors affecting containment of the disposed
and naturally-occurring brines. Sandstones, shales and mudstones of the
Dakota Formation, Kiowa Formation, and Cheyenne Sandstone comprise the
framework of the Great Plains aquifer.

Sandstone, siltstone, and shale of the Cedar Hills Formation comprise
the framework of the Cedar Hills aquifer. In the eastern half of the
area, the Cedar Hills aquifer directly underlies the Great Plains
aquifer, but in the western part relatively impermeable Jurassic and
Permian strata separate these two aquifer systems. Ground-water flow in
the Great Plains aquifer is generally from the deeper part of the
Western Kansas basin and southern outcrop areas towards the Smoky Hill
and Saline Rivers. Ground-water flow in the Cedar Hills aquifer is
subparallel to the flow in the Great Plains aquifer where they are in
contact.

Elsewhere in the western part of the area the configuration of the Cedar
Hills potentiometric surface has been affected by fluid injection.
Total dissolved solids concentrations of ground waters in the Cedar
Hills and lower Great Plains aquifers are generally greater than 20,000
mg/l and decrease vertically upward into the upper part of the Great
Plains aquifer.

Throughout the central Kansas area, the hydrologic data from the
monitoring sites indicates vertically upward movement of fluids from the
Cedar Hills and into the Great Plains aquifer. This is supported by the
similarity of general water chemistry, Br/Cl versus Cl mixing curves,
and the stable isotope data between aquifers. Hydrologic testing
suggest that upward movement of brines may be facilitated locally by
fractures. These results indicate that shallow underground disposal of
oil-field brines should be discontinued in the area of interconnection
between the Cedar Hills and Great Plains aquifers.